A study of the trace gas columns of O3, NO2 and HCHO over Africa in September 1997

Abstract

Retrievals of trace gas columns from the measurements of backscattered radiation by GOME (Global Ozone Monitoring Experiment) show that enhanced tropospheric columns of ozone (O3), nitrogen dioxide (NO2) and formaldehyde (HCHO), over the African continent occur frequently. This study focuses on the behaviour of trace gases over Africa in September 1997, a period impacted by the strongest known El Niño phase of the ENSO. It investigates our qualitative and quantitative understanding of the retrieved tropospheric trace gas column densities. The emissions of NOx and volatile organic compounds (VOC) from biomass burning, biogenic sources and lightning and their photochemical transformation have been investigated. By performing a trajectory analysis, the transport of air masses from the different emission regions was analysed and the potential atmospheric spatial distribution determined. BRemen's Atmospheric PHOtochemical model (BRAPHO) was applied to compute the chemistry along a large number of trajectories. From these results, tropospheric column amounts of O3, NO2 and HCHO were derived. Tropospheric trace gas columns retrieved from GOME measurements and those calculated are in reasonable agreement. Their general spatial extent was similar in the lower troposphere but the modeled trace gas columns in the upper troposphere were located south of the retrieved columns. We attribute this behaviour to uncertainties in the ERA-40 meteorological data in the upper troposphere. The significance of biomass burning and of biogenic emissions with respect to HCHO columns over Africa was investigated. The analysis reveals that the total amounts of HCHO generated over Africa during September 1997 as a result of biomass burning and biogenic emissions are similar. However the HCHO from biogenic sources has the highest specific columns and these are located close to their source. In comparison the HCHO from biomass burning is predicted to be produced and transported over a much wider area. Overall all the emission processes mix together to produce the plume of O3.

Remote sensed and in situ constraints on processes affecting tropical tropospheric ozone

“…Palmer et al (2003) showed that HCHO columns over North America are closely related to isoprene emissions, and exploited that relationship to infer continental isoprene emissions from the GOME HCHO columns. Meyer-Arnek et al (2005) found signals in the GOME HCHO columns from both biogenic and pyrogenic sources over Africa. Shim et al (2005) extended the approach of Palmer et al (2003) to infer global isoprene emissions, but found that large increases in biomass burning emissions were necessary to reconcile the GOME observations.…”

The empirical relationship between satellite-derived tropospheric NO<sub>2</sub> and fire radiative power and possible implications for fire emission rates of NO<sub>x</sub>

“…This is in good agreement with the findings of Yienger and Levy (1995), as they suggest much lower emission rates of NO x from soil microbial activity in tropical rain forests covering large areas in CSA. Meyer-Arnek et al (2005) have shown that a significant amount of TVC NO 2 could also be attributed to NO 2 production from lightning, as the lifetime of NO 2 is longer in the upper troposphere. In comparison to the dry season peak values of TVC NO 2 and FRP, the wet season values within ANE, ASE, CSA, and NAU are about two thirds smaller in magnitude.…”

The global lightning-induced nitrogen oxides source

“…The data and the analysis method are described in Richter et al (2005). Figure provided 2002a; Richter and Burrows, 2002;Duncan et al, 2003;Edwards et al, 2003;Kunhikrishnan et al, 2004;Savage et al, 2004;Choi et al, 2005;Irie et al, 2005;Jaeglé et al, 2005;Konovalov et al, 2005;Meyer-Arnek et al, 2005;Ma et al, 2006;van der A et al, 2006;van Noije et al, 2006). Figure 6 illustrates the improvement in spatial resolution provided by SCIAMACHY compared to GOME.…”